Odd thing with dual displays

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After loading Win10 on my W510 I had an odd problem with the dual diaplays on the Targus dock, both would show during boot but when Windows started only one would display. If I turned that one off, the other would display but not both. Screen Resolution>Detect only found the current one even if both were on.

 

Finally I happened to press WIN-P and selected "extend". Both screens came on and now Screen Resolution shows both.

 

Strange.